October 2013

Out on a limb: Beyond offloading

Lower extremity practitioners are constantly being reminded that what they think they know about injury prevention is only part of a bigger picture. This time it’s the diabetic foot care community’s turn to wonder just how much they have yet to learn about the mechanisms underlying ulceration.

For years, offloading has been one of the basic tenets of diabetic foot care management, based on the concept that elevated areas of peak plantar pressure are at increased risk of ulceration in patients with diabetes and particularly those with diabetic neuropathy.

It makes all kinds of sense intuitively. But there have been hints that maybe that wasn’t the whole story. Eyebrows were raised when researchers from the University of Amsterdam reported that, even when they confirmed that customized footwear reduced high-pressure areas to below 200 kPa (believed to be the threshold for ulceration), the 18-month reulceration rate in patients who wore the special shoes was not significantly lower than in a control group who wore standard therapeutic footwear (see “Footwear customized with pressure data fails to reduce diabetic reulceration rates”). Several previous studies that did not measure plantar pressures also found no effect of footwear on ulceration rates.

In the Dutch study, lower levels of compliance with shoe wear in the intervention group than in the control group might have contributed to the results, the researchers suggested. And that certainly makes sense. No intervention can be effective if patients don’t use it.

But now a study from the University of Washington in Seattle is suggesting that elevated peak plantar pressure is only predictive of ulcer risk in some regions of the foot, not all (see “Location, location: High pressures don’t predict all ulcers”). Of the ulcers that developed in 47 patients, only those in the metatarsal region developed in locations where peak plantar pressure was elevated at baseline (an average of 2.4 years earlier) compared to the rest of the foot. The hallux ulcers and heel ulcers emerged at sites where the peak plantar pressure at baseline was significantly lower than areas where ulcers did not develop. The mean baseline peak plantar pressure for hallux ulcer sites, in fact, was just 174 kPa, which is doubly counterintuitive.

If ulcers are developing in areas where plantar pressure is not elevated, then perhaps we should be less surprised that footwear customized to offload elevated areas of plantar pressure didn’t have the expected effect on ulceration rates.

The challenge for researchers now is to determine just what does contribute to ulceration in those cases. Shear forces? Foot type? Footwear design? Some combination of variables? Once other factors have been identified, footwear and foot orthoses can be better customized to address those issues in addition to issues of elevated plantar pressure in high-risk locations. The design of custom diabetic footwear is about to get a lot more interesting.

Offloading areas of elevated pressure will continue to be an important component of diabetic foot ulcer prevention. But it’s just one piece of what is turning out to be a much more complicated puzzle than once thought.